1. Field of the Invention
This invention relates to electrical interconnect apparatus. More particularly, the present invention relates apparatus for connecting a thermal inkjet pen and associated sensor to a printed circuit board.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Inkjet printer/plotters and desktop printers, such as those sold by Hewlett Packard Company, offer substantial improvements in speed over the conventional X-Y plotter. Inkjet printer/plotters typically include a pen having an array of nozzles. The pens are mounted on a carriage which is moved across the page in successive swaths. Each inkjet pen has heater circuits which, when activated, cause ink to be ejected from associated nozzles. As the pen is positioned over a given location, a jet of ink is ejected from the nozzle to create a pixel at a selected location. The mosaic of pixels thus created provides a composite image.
Inkjet technology is now well known in the art. See, for example, U.S. Pat. No. 4,872,027, entitled PRINTER HAVING IDENTIFIABLE INTERCHANGEABLE HEADS, issued Oct. 3, 1989, to W. A. Buskirk et al. and U.S. Pat. No. 4,965,593, entitled PRINT QUALITY OF DOT PRINTERS, issued Oct. 23, 1990, to M. S. Hickman, the teachings of which are incorporated herein by reference.
Recently, full color inkjet printer/plotters and desktop printers have been developed which comprise a plurality of inkjet pens of diverse colors. A typical color inkjet printer/plotter has four inkjet pens, one that stores black ink, and three that store colored inks, e.g., magenta, cyan and yellow. The colors from the three color pens are mixed to obtain various colors.
The pens are typically mounted in stalls within an assembly which is mounted on the carriage of the printer/plotter. The carriage assembly positions the inkjet pens and typically holds the circuitry required for interfacing to the heater driver circuits in the inkjet pens.
A carriage assembly typically includes four pen stalls to align the inkjet pens, four pen clamps to hold the inkjet pens in the pen stalls, a printed circuit board with circuitry for interfacing the heater circuits in the inkjet pens, and four separate flexible (flex) circuits interconnecting the printed circuit board with electrical contacts on the inkjet pens. The flex circuit is typically made with a polyester or polyamide material such as a Mylar or Kapton substrate onto which multiple conductors or traces are created. The use of four separate flex circuits has the disadvantages of: 1) high cost due to the need to manufacture and stock the multiple separate flex circuits; 2) difficulty of assembly because of the need to route the flex circuit in the carriage assembly and precisely align each of the flex circuits to each of the pen housings; 3) high cost of assembly because the separate flex circuits need to be separately interconnected with the printed circuit board; and 4) the need to provide separate grounds for each flex circuit.
Accordingly, there was a need in the art for an improved interconnect system that reduced cost, was easy to assemble and align, and provided ground plane sharing for all of the inkjet pens. This need was addressed by U.S. patent application entitled UNITARY INTERCONNECT SYSTEM FOR AN INKJET PRINTER, filed Apr. 30, 1993 by A. K. Wilson et al., Ser. No. 08/055,615 (Attorney Docket No. HP 1093126), the teachings of which are incorporated herein by reference. This application discloses and claims an advantageous flexible circuit for use with inkjet printers.
Unfortunately, this system provides for the interconnection of a single flex circuit to the printed circuit board. For some applications, there is a need to connect multiple flex circuits to a printed circuit board.
There are a number of conventional approaches to the interconnection of multiple flexible circuits to a printed circuit board. One conventional approach is to simply solder the additional flex circuit to the printed circuit board (PCB). This approach, however, has the disadvantages of: 1) low interconnect density, because it is necessary to maintain the spacing between interconnects to prevent solder bridging, and 2) the need to use Kapton, which is more tolerant to heat than Mylar. Parts are costly and fabrication is difficult and costly as well.
A second conventional approach is to solder a connector to the additional flexible circuit for connection with a connector soldered onto the PCB. This approach offers ease of assembly and disassembly, but requires a connector on both the printed circuit board and the flexible circuit. In addition it requires a stiffener to be attached to the flexible circuit to support the connector, which increases the cost. It also has the disadvantage of achieving only low interconnect density.
A third conventional approach uses high pressure to force the flat contacts of the additional flex circuits onto the contacts of a printed circuit board. This approach has the disadvantage that a rigid structure is required to support the high pressure. In addition, this approach is limited in that it requires the contacts on the PCB to be above the surface of the PCB. Often a coating such as a solder mask is applied over a PCB to protect the conductors from solder bridging and moisture, which leaves the exposed contacts on the PCB slightly recessed below the surface of the PCB, which makes connection with the contacts difficult. The high pressure approach also has the disadvantage of high cost.
A fourth conventional approach is to use spring fingers which are soldered onto the PCB. The flexible circuit is then pressed onto the spring fingers to make contact. This approach has the shortcomings of high cost and low interconnect density, because of the space requirements for soldering the spring fingers to the PCB.
Another technique that has been used is a called a zebra stripe and consists of a elastomer having alternating conductive and nonconductive elastomer sections. The zebra stripe is placed between a flexible circuit and a PCB and then the entire assembly is pressed together to complete the interconnect. This approach has the shortcomings of low interconnect density, high cost, and low reliability.
Accordingly, there is a need in the art for an improved interconnect system that provides for low cost, high density, high reliability connection of multiple flex circuits to a printed circuit board.